1. Field of the Invention
The present invention relates to a transmission circuit used in communication devices such as mobile phones and wireless LAN devices. The present invention particularly relates to a transmission circuit capable of operating with high linearity and high efficiency, and to a communication device using the transmission circuit.
2. Description of the Background Art
Communication devices such as mobile phones and wireless LAN devices are required to secure precision of an output signal and operate with low power consumption. Used in such communication devices is a transmission circuit for outputting a highly linear transmission signal, which operates with high efficiency. Hereinafter, a conventional transmission circuit will be described.
One of the conventional transmission circuits is, for example, a transmission circuit which uses a modulation method such as a quadrature modulation method to generate a transmission signal (herein after, referred to as a quadrature modulation circuit). Since the quadrature modulation circuit is well known, a description thereof will be omitted. A conventional transmission circuit, which is smaller in size and operates more efficiently than the quadrature modulation circuit, is a transmission circuit shown in Non-Patent Document 1, for example. FIG. 11 is a block diagram showing an exemplary structure of a conventional transmission circuit 500 disclosed in Non-Patent Document 1. In FIG. 11, the conventional transmission circuit 500 comprises a signal converter 501, an AM fixed fc filter 502, a PM fixed fc filter 503, a phase modulator 504, an amplifier 505, a power supply terminal 506, a power amplifier 507 and an output terminal 508.
I and Q signals are inputted to the signal converter 501. The signal converter 501 converts the I and Q signals into an amplitude signal and a phase signal, respectively, and outputs these signals. The amplitude signal is inputted to the AM fixed fc filter 502. The AM fixed fc filter 502 removes a high-frequency component from the amplitude signal, by using a fixed cutoff frequency (AM_fc). The amplitude signal outputted from the AM fixed fc filter 502 s inputted to the amplifier 505. The amplifier 505 supplies the power amplifier 507 with a voltage which is proportional to the inputted amplitude signal. This improves current-driven capability of the power amplifier 507. Note that, the amplifier 505 is supplied with a DC voltage from the power supply terminal 506.
The phase signal is inputted to the PM fixed fc filter 503. The PM fixed fc filter 503 uses a fixed cutoff frequency (PM_fc) to remove a high-frequency component from the phase signal. The phase signal outputted from the PM fixed fc filter 503 is inputted to the phase modulator 504. The phase modulator 504 phase-modulates the phase signal to output a high-frequency phase-modulated signal. The high-frequency phase-modulated signal is inputted to the power amplifier 507. The power amplifier 507 amplifies the high-frequency phase-modulated signal by using the voltage supplied from the amplifier 505, and outputs a resultant signal as an amplified signal. This amplified signal is outputted from the output terminal 508 as a transmission signal. The transmission circuit 500 as described above is referred to as a polar modulation circuit.
Here, based on the cutoff frequencies of the AM fixed fc filter 502 and the PM fixed fc filter 503, there are trade-off relationships among noise, distortion (ACLR) and EVM of the transmission signal. Provided below is an exemplary case where a UTMS modulation signal stipulated by 3GPP (3rd Generation Partnership Project: 3rd generation (3G) mobile unit communication system) standards is transmitted. FIG. 12A shows a reception band noise at a frequency which is 45 MHz away from a frequency of a transmission carrier signal contained in the transmission signal outputted from the conventional transmission circuit 500. FIG. 12B shows the distortion of the transmission signal (i.e., ACLR at a frequency 10 MHz away) outputted from the conventional transmission circuit 500. FIG. 12C shows the EVM of the transmission signal outputted from the conventional transmission circuit 500. In FIGS. 12A to 12C, the horizontal axis represents the cutoff frequency (AM_fc) of the AM fixed fc filter 502 which removes a high-frequency component from the amplitude signal, and the longitudinal axis represents the cutoff frequency (PM_fc) of the PM fixed fc filter 503 which removes a high-frequency component from the phase signal.
As shown in FIG. 12A, the noise of the transmission signal is decreased in accordance with a decrease in the cutoff frequency (AM_fc) of the AM fixed fc filter 502 and in the cutoff frequency (PM_fc) of the PM fixed fc filter 503. Also, as shown in FIG. 12B, the distortion (ACLR) of the transmission signal is improved in accordance with an increase in the cutoff frequency (AM_fc) of the AM fixed fc filter 502 and in the cutoff frequency (PM_fc) of the PM fixed fc filter 503. Further, as shown in FIG. 12C, the EVM of the transmission signal is improved in accordance with an increase in the cutoff frequency (PM_fc) of the PM fixed fc filter 503.
In the conventional transmission circuit 500, the cutoff frequency (AM_fc) of the AM fixed fc filter 502 and the cutoff frequency (PM_fc) of the PM fixed fc filter 503 are predetermined in consideration of a balance among the noise, distortion (ACLR) and EVM of the transmission signal such that an optimal transmission signal is outputted.
Non-Patent Document 1: F. H. Raab et al., “High-Efficiency L-Band Kahn-Technique Transmitter”, 1998, IEEE MTT-S Int. Microwave Symp. Dig.
However, the conventional transmission circuit 500 has a problem that since the cutoff frequency (AM_fc) of the AM fixed fc filter 502 and the cutoff frequency (PM_fc) of the PM fixed fc filter 503 are fixed, the balance among the noise, ACLR and EVM of the transmission signal is disrupted when, e.g., an output power of the transmission signal changes. For this reason, the conventional transmission circuit 500 does not always output a highly-linear low-noise transmission signal.